WO2004083881A1

WO2004083881A1 - Magnetoresistive sensor, comprising a ferromagnetic/antiferromagnetic sensitive element

Info

Publication number: WO2004083881A1
Application number: PCT/FR2004/000574
Authority: WO
Inventors: Michel Hehn; Alain Schuhl; Grégory MALINOWSKI; Christophe Nicot; Christophe Duret
Original assignee: S.N.R. Roulements
Priority date: 2003-03-14
Filing date: 2004-03-10
Publication date: 2004-09-30
Also published as: FR2852400A1; JP5731873B2; EP1604220A1; JP2006520538A; US7602178B2; JP2011159988A; FR2852400B1; JP2015159312A; KR101100841B1; JP2013165295A; US20070159164A1; KR20050109996A

Abstract

The invention relates to a magnetoresistive sensor for a magnetic field, comprising a stack (1) of a reference element (2), a separation element (3) and an element (4) sensitive to magnetic fields, whereby the reference element (2) and the sensitive element (4) have respectively a first and a second magnetic anisotropy (5, 6) in a first and a second direction. The sensitive element (4) comprises the superimposition of a layer of a ferromagnetic material (FM1) and a layer of an antiferromagnetic material (AF1) arranged to give a magnetic moment (10), of which the component in the direction of the field for measurement varies reversibly as a function of the intensity of the magnetic field for measurement and linearly within an adjustable field range. The invention further relates to use of such a sensor.

Description

Maqnétoresistif sensor including a ferromagnetic / antiferromaαnétique sensitive element

The invention relates to a magnetoresistive magnetic field sensor and the use of such a sensor for measuring the intensity of a magnetic field.

Historically, magnetoresistive sensors exploit the variation of electrical resistance of a single magnetic material which is induced by the variation of the magnetic field to be measured. This is the operating principle of anisotropic magnetoresistance sensors. However, the variation in resistance is small. Since the discovery of giant magnetoresistance (in 1988) and tunnel magnetoresistance at room temperature (1995), other sensor architectures have been imagined with resistance variations of more than 50% at room temperature.

These sensors include the stack of a reference magnetic element, a separation element and a magnetic element sensitive to the magnetic field, said stack being arranged to present a variation in electrical resistance as a function of the magnetic field to be measured.

In particular, the stack can comprise two magnetic structures respectively forming a reference element and a sensitive element which are separated by the separation element. In this configuration, the orientation of the magnetic moment of the reference element is arranged to be unchanged by the action of the magnetic field to be measured, while that of the sensitive element is modifiable by the action of said field.

When the separating element is electrically conductive (a metallic or semi-conductive layer for example), the sensor exploits the giant magnetoresistance which translates the dependence of the current as a function of the relative orientation of the magnetizations of the magnetic structures. And, when the separating element is electrically insulating, the sensor uses the tunnel magnetoresistance which depends on the structure of interface bands of the spin up and down electrons and which for a given spin channel depends on the relative orientation of their magnetization. These sensors are very sensitive and can a priori be intended for magnetic field detection, the amplitude of which can vary by several orders of magnitude.

To obtain an efficient magnetoresistive sensor, it is necessary to control the relative orientation of the magnetic moments of the magnetic structures, so as to be able to correlate the variation in electrical resistance with the magnetic field to be measured. In particular, a perpendicular orientation of the magnetic anisotropy axis of the reference element relative to that of the sensitive element makes it possible to linearize the output of the sensor in order to obtain an easily exploitable measurement signal.

Document FR-2 809 185 describes a sensor in which the sensitive element comprises a layer of ferromagnetic material, the magnetic anisotropy of which comes from the form energy, and the reference element comprises the superposition of a layer of material ferromagnetic and a layer of antiferromagnetic material whose anisotropy results from the exchange between these two layers. According to this document, the form energy is therefore used to obtain the sensitive element, and the exchange anisotropy is used to obtain the reference element, that is to say to obtain a fixed magnetic moment in function of this field.

This solution has several disadvantages both from the point of view of the sensor design and that of the performance of the measurement obtained.

Regarding the design, the use of form energy to induce the anisotropy of the sensitive element proves difficult and costly to implement. Indeed, as explained in document FR-2 809 185, this requires the use of disoriented Si (111) vicinal surfaces, but this substrate is particularly expensive and difficult to use industrially. Indeed, annealing at high temperature (900 ° C) with a slow decrease in temperature is necessary to obtain the accumulation of the steps necessary for the observation of the shape anisotropy. In addition, this use requires a particular direction of anisotropy, which is detrimental to the modularity of the sensor. In addition, these substrates are not suitable for integration of the sensitive element on a signal processing ASIC.

Regarding measurement performance, it turns out that the range of use of the known sensor is difficult to adapt, and that in any event it remains relatively limited. In particular, this range of use depends on the size of the sensor, which also affects the modularity of the sensor. In addition, the arrangement of the antiferromagnetic layer on the upper part of the stack poses problems of reliability of the measurement. Indeed, it has been shown that a texture of the antiferromagnetic layer is necessary for strong blocking and therefore for an operating range at high temperature. However, when the antiferromagnetic layer is placed on top of an amorphous insulating layer, the texture is lost, the blocking is less and the sensor no longer works for temperatures slightly above ambient temperature.

To resolve all of these drawbacks, the invention proposes a magnetoresistive sensor in which the magnetic anisotropy of the sensitive element is induced by the exchange which exists at an interface between a layer of ferromagnetic material and a layer of antiferromagnetic material.

To this end, and according to a first aspect, the invention provides a magnetoresistive magnetic field sensor comprising a stack of a reference element, a separation element and an element sensitive to the magnetic field, in which the reference element and the sensitive element respectively have a first and a second magnetic anisotropy in a first and a second direction. The sensitive element comprises the superposition of a layer of ferromagnetic material and a layer of antiferromagnetic material which is arranged to obtain a magnetic moment, the component of which is oriented in the direction of the field to be measured. varies linearly and reversibly as a function of the intensity of the magnetic field to be measured, and linearly within an adjustable field range.

According to a second aspect, the invention proposes the use of such a sensor for measuring the intensity of a magnetic field, in which the direction of anisotropy of the reference element is arranged parallel to the direction of the magnetic field to be measured.

Other objects and advantages of the invention will appear during the description which follows, given with reference to the appended drawings, in which:

- Figures 1 and 2 are perspective views schematically showing respectively a first and a second embodiment of a stack of layers disposed on a substrate for the production of a sensor according to the invention;

- Figure 3 is a diagram of the magnetic configuration of the anisotropy axes, magnetizations and the magnetic field to be measured in the stacks according to Figures 1 or 2;

FIGS. 4a and 4b represent the variation, as a function of the magnetic field to be measured, of the magnetization respectively of the sensitive element and of the reference element according to the configuration of FIG. 3 and for the stack of FIG. 1 ;

FIGS. 5a and 5b represent the variation, as a function of the magnetic field to be measured, of the magnetization respectively of the sensitive element and of the reference element according to the configuration of FIG. 3 and for the stack of FIG. 2 ;

FIG. 6 represents the variation, as a function of the magnetic field to be measured, of the electrical resistance of the junction which results from the variations in magnetizations shown in FIGS. 4a and 4b; FIG. 7 represents the variation, as a function of the magnetic field to be measured, of the electrical resistance of the junction which results from the variations in magnetizations shown in FIGS. 5a and 5b;

FIG. 8 illustrates the variations in the electrical and magnetic sensitivities of a sensor according to the invention as a function of the temperature;

FIG. 9 illustrates the variation of its total sensitivity with temperature;

FIG. 10 illustrates the variation of the total sensitivity of a sensor optimized with the temperature.

The property which interests us here more particularly is the response obtained when a ferromagnetic material FM1 and an antiferromagnetic material AF1 have a common interface when the field is applied perpendicular to the magnetic axis exhibiting exchange. In this case, the process of reversing the magnetization by nucleation and propagation of walls (reversal when the field is applied along the magnetic axis exhibiting exchange) is replaced by the reversible rotation of the magnetization (reversal when the field is applied perpendicular to the magnetic axis with exchange). The hysteretic behavior is then replaced by the reversible behavior of FIG. 4a. In addition, in a fairly wide range of fields, the signal is linear.

Formally, the slope of the response of the magnetization with the applied field is given by:

where M _s is the saturation magnetization of the ferromagnetic layer FM1, t _F is the thickness of the ferromagnetic layer FM1, KF is the anisotropy constant of the ferromagnetic layer FM1 and J is the coupling existing between the ferromagnetic layer and the antiferromagnetic layer. When KF = 0, it is possible to know analytically the component of the magnetization of the layer FM1 in the direction of the applied field is

Thus the creation of a common ferromagnetic / antiferromagnetic interface induces a magnetic anisotropy axis, the direction of which is controllable in the ferromagnetic layer. The low amplitude magnetic field response is reversible with a slope, and therefore a sensitivity of the future sensor, adjustable by the function of M _s , t _F and J.

The invention relates to a magnetoresistive magnetic field sensor which comprises a stack 1 of a reference element 2, of a separation element 3 and of an element sensitive to the magnetic field. The reference element 2 and the sensitive element 4 respectively have a first 5 and a second 6 magnetic anisotropy in a first and a second direction.

This type of sensor is arranged so that, under the effect of the magnetic field to be measured, the direction of the magnetization 10 of the sensitive element 4 varies relative to that of the reference element 2, which induces a variation electrical resistance of the stack 1 as a function of the intensity of said field.

According to a first embodiment, the separation element 3 comprises a layer S of an electrically insulating material, for example based on oxidized and / or nitrated aluminum, oxidized gallium, oxidized tantalum, oxidized magnesium, titanate of oxidized strontium. The magnetoresistive sensor then exploits the tunnel magnetoresistance properties of the junction formed by the two magnetic elements 2, 4 separated by the insulating layer S. In this embodiment, the resistance measurements are carried out perpendicular to the plane of the layer S.

According to a second embodiment, the separation element 3 is formed of a layer S of electrically conductive material, for example based on metals such as copper or based on semiconductors. The magnetoresistive sensor then exploits the giant magnetoresistance properties of the "spin valve" formed by the two magnetic elements 2, 4 separated by the conductive layer S. In this embodiment, the resistance measurements are carried out either perpendicular to the plane of the layer S either parallel to him.

In these two embodiments, the magnetoresistive effect leads to a variation in the electrical resistance of the stack 1 as a function of the magnetic field to be measured, said variation being used in an electronic processing circuit to obtain the intensity of said field. In particular, the exploitation of the resistance variation is facilitated by providing that, in the absence of a magnetic field to be measured, the first anisotropy 5 is perpendicular to the second anisotropy 6.

In relation to FIG. 1, a first embodiment of the stack 1 is described which comprises a layer of ferromagnetic material FM2 as a reference element 2, and the superposition of a layer of a ferromagnetic material FM1 and a layer of an antiferromagnetic material AF1 as a sensitive element 4. The ferromagnetic materials FM1, FM2 are for example based on cobalt, iron, nickel or an alloy of these materials. The ferromagnetic materials of the reference element 2 and of the sensitive element 4 can be of identical or different nature depending on the characteristics desired for the sensor. The antiferromagnetic material can be based on IrMn, FeMn, PtMn, NiMn or other manganese-based compounds. When a ferromagnetic material and an antiferromagnetic material have a common interface, it is possible to observe an effect called "exchange bias" which manifests itself mainly by a displacement in magnetic field of the hysteresis cycle. The ferromagnetic layer FM1 then has an anisotropy direction 6 imposed by the antiferromagnetic material AF1. This direction of anisotropy 6 has the advantage of being controllable, either by saturating the magnetization of the ferromagnetic layer FM1 during the deposition of the layer AF1, or by a heat treatment under magnetic field after deposition where the sample is heated at a temperature higher than the blocking temperature of the antiferromagnetic material AF1 before being cooled below this temperature. During this cooling, it must be ensured that the magnetization of the ferromagnetic layer FM1 is saturated in the direction desired for the anisotropy of the layer.

The stack 1 is deposited on a substrate 7, for example made of silicon or glass, the layer of antiferromagnetic material AF1 being disposed on the substrate. To do this, one can use a sputtering technique under vacuum which allows to successively deposit thin layers of desired materials. Regarding the deposition of a layer of oxidized aluminum, provision may be made to deposit an aluminum layer by sputtering under vacuum and then to oxidize this layer under oxygen.

To limit the formation of defects in the layer of antiferromagnetic material AF1, it can be envisaged to deposit on the substrate 7 a buffer layer, for example an amorphous film of tantalum 8, which is intended to improve the state of the surface on which the antiferromagnetic material AF1 is arranged.

In this embodiment, the anisotropy 5 of the reference element 2 is obtained either by depositing the layer of ferromagnetic material FM2 under magnetic field so as to orient this anisotropy 5 in the direction of the applied magnetic field, or by inducing a shape anisotropy in the layer of ferromagnetic material FM2, for example by providing that the reference element 2 has a larger dimension in the direction of the anisotropy 5. The reference element 2 is arranged to have a higher coercive field than the range of field to be measured. Thus, by applying a magnetic field, it is possible to induce a modification of the orientation of the magnetic moment of the sensitive element 4 without modifying the magnetic moment of the reference element 2.

By way of example, the following magnetic tunnel junction has been produced:

Glass / Ta (5 nm) / Co (10 nm) / IrMn (10 nm) / Co (10 nm) / AlOx / Co (2 nm) / Co ₈₀ Pt2o (5 nm) / Pt (4nm)

Glass constitutes the substrate and the Ta / Co bilayer is the buffer layer. The sensitive element is composed of the IrMn (10 nm) / Co (10 nm) bilayer. The Co (2 nm) / Co ₈₀ Pt.2o (5 nm) reference element consists of cobalt with added platinum to increase the coercive field. The Pt layer (4nm) is a protective layer.

The layers were deposited by sputtering at room temperature under a base pressure less than 5.10 ^"7 mbar. The Argon pressure during the deposition was 5.10 ^{" 3} mbar.

To obtain the insulating layer (AlOx), the oxidation was carried out after deposition of a metallic layer of 1.3 nm using a continuous luminescent discharge at 300 W for 35 seconds under pure oxygen plasma at 10 ^"1 mbar in a sputtering enclosure. The sample was transferred to this enclosure without breaking the vacuum.

After growth, the sample was annealed for 30 minutes at 200 ° C in the presence of a magnetic field of 300 Oe in order to establish the "exchange bias" in the IrMn layer and to orient the direction of anisotropy of this layer perpendicular to the direction of anisotropy of the layer Co (2 nm) / Co ₈₀ Pt ₂ o (5 nm) / Pt (4nm).

The conformation of the junction was carried out in a known manner by UV lithography and ion beam etching.

According to the second embodiment shown in FIG. 2, the reference element 2 comprises the superposition of a layer of a ferromagnetic material FM2 and a layer of an antiferromagnetic material AF2, and the sensitive element 4 is similar to that shown in Figure 1. This embodiment provides greater stability of the reference element 2 vis-à-vis the magnetic field to be measured (Figure 5b).

In this embodiment, the sensor therefore comprises the stack AF1 / FM1 / S / FM2 / AF2, the antiferromagnetic materials AF1 and AF2 having blocking temperatures, respectively T1 and T2, which are different, for example with T1> T2 . To obtain the magnetic configuration shown in FIG. 3, one can proceed as follows:

- The stack 1 is annealed at a temperature T> T1 under magnetic field so as to induce anisotropy respectively in the sensitive element 4 and in the reference element 2 which is parallel to the applied magnetic field; then

the stack is annealed at a temperature T of between T1 and T2 under a magnetic field perpendicular to that applied in the previous step, so as to induce anisotropy 5 in the reference element 2 which is parallel to the applied magnetic field and therefore perpendicular to the anisotropy 6 of the sensitive element 4.

In these two embodiments, the sensitive element 4 is arranged so that its magnetic moment 10 varies according to the magnetic field to be measured, and the reference element 2 is arranged so that the direction and the direction of its magnetic moment 9 are fixed according to the magnetic field to be measured. These characteristics are obtained, depending on the intensity of the magnetic field to be measured, by varying the nature of the materials used and / or the thickness of the different layers. In particular, the thicknesses of the layers may be of the order of 10 nm and be arranged to obtain the desired junction, tunnel or giant magnetoresistance, and this within the intensity range of the magnetic field to be measured.

FIG. 3 represents a possible magnetic configuration for the anisotropy axes 5, 6 and the magnetizations 9, 10 respectively of the reference 2 and sensitive elements 4. In this configuration, under zero magnetic field, the magnetic moments 10, 9 are perpendicular . When a magnetic field to be measured 11 is applied in a fixed direction parallel to the direction of the anisotropy 5 of the reference element 2, this results in a rotation of the magnetic moment 10 of the sensitive layer 4 (towards a position 10 '), while the magnetization 9 of the reference layer 2 remains fixed.

As can be seen in FIGS. 4a and 5a, the variation in the magnetization of the sensitive element 4 in the direction of the applied field is linear over a wide range of variation in the intensity of the field to be measured (between -50 and +50 Oe in FIGS. 4a and 4b) while the magnetization of the reference element 2 remains constant over this range (FIG. 4b and 5b). As regards the magnetization of the reference element 2 (FIG. 4b), the coercive field, which corresponds to the reversal of the magnetization under the effect of the field to be measured, is of the order of 100 Oe (FIG. 4b) or 300 Oe (Figure 5b), well beyond the range of linearity of Figure 4a.

Consequently, a variation in the resistance of a stack 1 according to the invention as shown in FIGS. 6 and 7 is obtained which comprises as an important characteristic the fact of having a linear and reversible response over a large range of intensity. field to be measured (between -50 and +50 Oe). This law of variation can therefore be used in a particularly simple way in an electronic processing circuit to obtain the intensity of the magnetic field as a function of the resistance of the stack 1, since the variation of the resistance is linear as a function of the intensity of the magnetic field to be measured.

It can also be shown that the total sensitivity S of the sensor breaks down into an electrical sensitivity S _e and a magnetic sensitivity S, _n so that S≈

S _e x S _m - with S _e = (R _P - R _Λ p) l2 and S _m = l / ff "where R _P and R _AP are the resistances of the junction for the parallel and antiparallel alignments respectively of the magnetizations of l 'reference element and sensitive element e \ H _ex = JI (M _s t _F ) is the exchange field operating in the IrMn / Co bilayer.

The magnetic sensitivity and the electrical sensitivity were measured independently on the sample described above. For this purpose, the resistance as a function of the field was measured at different temperatures with an applied field parallel to the direction of anisotropy of the antiferromagnetic layer of the sensitive element in order to have unambiguous access to (R _P -R _ÂP ) I2 and H _ex . Figure 8 illustrates the results obtained. In this figure, the curve (•) represents the electrical sensitivity, and the curve (o) represents the inverse of the magnetic sensitivity, for temperatures up to 430 K.

Magnetic sensitivity varies linearly with temperature. Surprisingly, the same is true for electrical sensitivity. Thus, the total sensitivity also varies linearly (in this case it increases, as shown in Figure 9) with temperature.

In fact, a fine analysis of Figure 8 shows that the resistance of the junction varies as a function of the temperature according to a law

R (T) = R (0) (lC- ~ T ² ) Φ

where C is a constant, d is the thickness of the insulating layer, and Φ is the height of the barrier of the junction in eV. Consequently, it is possible to fix the slope of S _e by modifying the parameters d and Φ of the barrier, and in particular its thickness. For a junction with a given barrier height, it is therefore possible to determine the thickness of the insulating layer so that the slope of S _e as a function of temperature compensates for that of S _m and that the total sensitivity of the sensor is independent of temperature.

The variation as a function of the temperature of the total sensitivity of the sample described above is illustrated in FIG. 10 where it can be seen that it is practically non-existent.

Claims

1. Magnetoresistive magnetic field sensor comprising a stack

(1) a reference element (2), a separation element (3) and a sensitive element (4) to the magnetic field, in which the reference element

(2) and the sensitive element (4) respectively have a first and a second magnetic anisotropy (5, 6) in a first and a second direction, said sensor being characterized in that the sensitive element (4) comprises the superposition a layer of a ferromagnetic material (FM1) and a layer of an antiferromagnetic material (AF1) which is arranged to obtain a magnetic moment (10) whose component oriented in the direction of the field to be measured varies reversibly in function of the intensity of the magnetic field to be rhesured, and linearly in an adjustable field range.

2. Sensor according to claim 1, characterized in that the first anisotropy (5) is perpendicular to the second anisotropy (6).

3. Sensor according to claim 1 or 2, characterized in that the reference element (2) comprises a layer of ferromagnetic material (FM2) having a direction and a direction of magnetization (9) fixed according to the field magnetic to measure.

4. Sensor according to claim 1 or 2, characterized in that the reference element (2) comprises the superposition of a layer of a ferromagnetic material (FM2) and a layer of an antiferromagnetic material (AF2) which is arranged to obtain a direction and a direction of magnetization (9) fixed according to the magnetic field to be measured.

5. Sensor according to claim 4, characterized in that the blocking temperature of the antiferromagnetic material (AF2) of the sensitive element (2) is different from the blocking temperature of the antiferromagnetic material (AF1) of the reference element ( 4).

6. Sensor according to any one of claims 1 to 7, characterized in that the layer of antiferromagnetic material (AF1) of the sensitive element (4) is disposed on a substrate (7).

7. Sensor according to claim 8, characterized in that the substrate (7) comprises a layer (8) of a material which is intended to improve the state of the surface on which the antiferromagnetic material (AF1) is disposed.

8. Sensor according to any one of claims 1 to 5, characterized in that the separation element (3) is formed of a layer (S) of electrically conductive material.

9. Sensor according to any one of claims 1 to 5, characterized in that the separation element (3) comprises a layer (S) of an electrically insulating material.

10. Sensor according to claim 9, characterized in that its sensitivity is substantially independent of the temperature.

11. Sensor according to claim 10, characterized in that the thickness of the separation element is such, as a function of the barrier height of the magnetic tunnel junction formed by the stack of the reference element, of the the separation element and the sensitive element, that the variation in temperature of the electrical sensitivity of the sensor substantially compensates for the variation in temperature of its magnetic sensitivity.

12. Use of a sensor according to any one of claims 1 to 11 for measuring the intensity of a magnetic field, in which the direction of anisotropy (5) of the reference element (2) is arranged parallel to the direction of the magnetic field to be measured.